Microfluidic pumping and dispensing of liquid chemical reagents is the subject of three U.S. Pat. Nos. 5,585,069; 5,593,838; and 5,603,351, all assigned to the David Sarnoff Research Center, Inc., and hereby incorporated by reference. The system uses an array of reservoirs, with connecting microchannels and reaction cells etched into a substrate. Electrokinetic pumps comprising electrically activated electrodes within the capillary microchannels provide the propulsive forces to move the liquid reagents within the system. The electrokinetic pump, which is also known as an electroosmotic pump, has been disclosed by Dasgupta et al., see "Electroosmosis: A Reliable Fluid Propulsion System for Flow Injection Analyses", Anal. Chem. 66, pp 1792-1798 (1994). The chemical reagent solutions are pumped from a reservoir, mixed in controlled amounts, and then pumped into a bottom array of reaction cells. The array may be decoupled from the assembly and removed for incubation or analysis.
The above described microfluidic pumping can be used as a printing device. The fluids pumped become ink solutions comprising colorants such as dyes or pigments. The array of reaction cells may be considered ink delivery chambers to be used for picture elements, or pixels, in a display, comprising mixtures of pigments having the hue of the pixel in the original scene. When contacted with paper, the capillary force of the paper fibers draws the dye from the cells and holds it in the paper, thus producing a paper print, similar to a photograph, of the original scene.
A problem that exists with the microfluidic printing is the difficulty in controlling the amount of inks transferred from the ink delivery chambers to a receiver. During printing, the ink meniscus in the ink mixing pixel chambers is brought into contact with the receiver medium. The inks are absorbed by the receiver medium by the capillary action of the fibers or pores in the receiver medium. Since the capillary force in the receiver medium is typically much stronger than the holding strength of the microchannels in the microfluidic printing device, the ink transfer needs to be stopped at just the right time to prevent excess inks from being continually drawn from the microchannels in the microfluidic printing device. Furthermore, the amount of ink transfer varies as a function of temperature, because the ink viscosity is temperature dependent. As it is well know to those skilled in the art, excessive ink transfer to the receiver medium causes severe coalescence or smearing of the ink on the receiver, which produces visible image artifacts and lowers the printing resolution. Excess ink transfer also causes blending between inks of different colors which produces image defects and variability in color balance.